Previously, if the header was present, we'd proceed even if the
function wasn't there.
Easy fix for bug 19161. A better fix would involve trying harder to
find libscrypt_scrypt.
Add a new and slow unit test that checks if libscrypt_scrypt() and
EBP_PBE_scrypt() yield the same keys from test vectors.
squash! Assert interoperability betweeen libscrypt and OpenSSL EBP_PBE_scrypt().
squash! Assert interoperability betweeen libscrypt and OpenSSL EBP_PBE_scrypt().
squash! Assert interoperability betweeen libscrypt and OpenSSL EBP_PBE_scrypt().